A hybrid system working machine is constituted such that an averaged part of the required electric power is supplied from a generator, and when the generator is deficient in the electric power to be supplied, the shortage is supplemented by the electric power stored from a power storage means, whereas when the electric power is supplied in a relatively greater amount from the generator, the surplus electric power is stored (charged) in the power storage means.
Accordingly, the power storage means repeats the charging/discharging, changing the input/output in accordance with a variation in a load.
FIG. 6 shows a general trend of the power variation in a conventional non-hybrid system hydraulic excavator (relationship between the time and a change in the power which is determined from data for an actual machine). Regarding the property of a hydraulic excavator receiving a strong variation in the load, it is shown that either the total power from actuators or the power from a hydraulic pump as a power source for the actuators varies significantly in a short time.
On the other hand, FIG. 7 shows a trend (simulation) of a variation in the power for various leveling works, when a hybrid system excavator is employed.
In the case of such a hybrid system excavator, it is constituted such that the averaged power in the required power supplied from a power source, as shown by the solid line in FIG. 7, is supplied from a generator (one dot line), and that when the power is supplied in a relatively smaller amount from the generator, the shortage from the averaged power is supplemented by the power supplied from a power storage means, whereas when the power is supplied in a relatively larger amount from the generator, the surplus power is stored in the power storage means.
As a result, the output from the power storage means frequently and significantly varies, as indicated by the broken line in FIG. 7, thereby repeating inputting (charging) above the level of 0 kW and outputting (discharging) below the level thereof in a short time.
On the other hand, the excavator is constituted such that an upper rotating body is mounted onto a crawler type lower traveling body, and a excavating apparatus as a main working apparatus is mounted onto the upper rotating body, thereby making it possible either to carry out normal works (excavating, leveling and others) with the aid of the excavating apparatus, or to move in a site in the self-traveling manner with the aid of a crawler type traveling apparatus mounted onto the lower traveling body.
The movement is normally carried out in a small distance, so that it can be achieved by only the power from an engine. However, in a possible case when the excavator has to self-travel at a high speed in a relatively long distance, an assistance of the power storage means is further required.
In such a case, the power storage means has to have a greater capacity, which ensures an output for a long period, and therefore a battery (secondary battery, such as lead-acid battery, nickel hydrogen battery or the like) can be preferably employed for the power storage means.
On contrary, such a battery is operated with the aid of the chemical reaction between an electrolyte and electrodes, and therefore this causes the lifetime to become extremely short, when it is used in a working machine, such as an excavator, wherein the charging/discharging is frequently repeated due to a significant variation in the load. This eventually necessitates the exchange of the battery with high frequency, and leads to an increased cost in operation as well as to a disadvantage resulting from the environment problem due to the abandonment of batteries.
On the other hand, the employment of a capacitor (electrical double layer condenser) for such a power storage means provides a longer lifetime, even if the charging/discharging is frequently repeated. This is due to the fact that the electric charges are physically stored in the capacitor. Nevertheless, a reduced storage capacity provides a rapid consumption of charges, thereby making it difficult to travel at high speed in a long distance as described above. In the case of an empty storage, the movement or traveling has to be carried out, using only the output from the engine, so that the traveling speed is reduced and a long time is required for the movement.
As described in Japanese Unexamined Patent Application Publication No. 2000-295717, when an excavator is constituted such that it includes two type condensers, a battery and a capacitor, and that the condensers may be selectively used in accordance with the working conditions, the above-mentioned problems are solved. In this case, however, the cost of manufacturing the condensers extremely increases, along with increased complexity in the control, so that such a system is not available in the practical usage.